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The giant magnetoimpedance (GMI) effect in Co75Fe4.2Si8B12Nb0.8 melt extraction wires annealed in a vacuum furnace at 450°C holding for 10min, 20min, 25min, 30min and 40min, respectively, is investigated. Strong GMI effect is observed at a typical frequency of 37MHz. The observation on the GMI effect indicates that the anisotropy field dominated by the anneal treatment is vital to the GMI ratio △Z/Z(%) and its field sensitivity. The improvement on GMI effect when anneal time is less than 20min is ascribed to the stress release, decreased anisotropy field and increased permeability. The decrease of GMI effect when anneal time is over 25min results from nanocrystalline phase grow up inducing the crystalline anisotropy field increase. Suitable annealed wires possess large GMI ratio (855%) and little anisotropy field achieving high field sensitivity to 151%/Oe. These results indicate these Co-based melt extraction wires are excellent for using in novel field sensors applications.

Due to the extremely weak intensity of the biomagnetic field and the serious interference from the environmental magnetic field, the detection of the biomagnetic field becomes such challenging work. After analyzing the deficiencies in the current biomagnetic field sensors, this paper proposes and realizes a high-sensitivity magnetic field sensor, based on the giant magneto-impedance (GMI) effect. Taking advantage of the miniaturized magnetic probe, the multistage multiple amplification and the multiband interference suppression, our sensor mainly makes three achievements: the pT level magnetic resolution, the ability to detect the muscle magnetic field without the magnetic shielding and the resistibility to a small-range wobbling in the state of working, which makes it possible to detect the biomagnetic field by wearable sensors under natural conditions.

Naked Co_68.25Fe_4.5Si_12.25B₁₅ amorphous wires of 67μm, 56μm, 52μm, 47μm and 31μm in diameter are produced by melt extraction method. Their giant magneto impedance (GMI) effect is investigated at frequencies from 0.1MHz to13MHz. Significant diameter dependence of GMI effect is studied. Thicker wires exhibit strong GMI effect and have clear characteristic frequencies at which their impedance ratio ΔZ/Z are largest. Largest impedance response is obtained in 67μm wires with the ΔZ/Z of 442% and field sensitivity of 71.5%/Oe. Wires of 31μm in diameter show increasing ΔZ/Z as frequency and have a steady field sensitivity of 30.7-33.6%/Oe in a wide frequency range from 3MHz to 13MHz. The different frequency dependence of GMI effect is discussed in the light of the skin effect. These amorphous wires are suitable for applications in high performance field sensors and can fit different demand.